1. Field of the Invention
The present invention relates to an apparatus for manufacturing a semiconductor device having a circuit comprising thin film transistors. For example, the invention relates to an apparatus for manufacturing an electro-optical device, typically a liquid crystal display device, and the structure of electric equipment mounted with such an electro-optical device as a component. Note that throughout this specification, the semiconductor device indicates general devices that may function by use of semiconductor characteristics, and that the above electro-optical device and electric equipment are categorized as the semiconductor device.
2. Description of the Related Art
In recent years, the technique of crystallizing and improving the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having a crystallinity which is polycrystalline or microcrystalline, but is not a single crystal), in other words, a non-single crystal silicon film, formed on an insulating substrate such as a glass substrate by laser annealing, has been widely researched. Silicon film is often used as the above semiconductor film.
Comparing a glass substrate with a quartz substrate, which is often used conventionally, the glass substrate has advantages of low-cost and great workability, and can be easily formed into a large surface area substrate. This is why the above research is performed. Also, the reason for preferably using a laser for crystallization resides in that the melting point of a glass substrate is low. The laser is capable of imparting high energy only to the non-single crystalline film without causing much change in the temperature of the substrate.
The crystalline silicon film is formed from many crystal grains. Therefore, it is called a polycrystalline silicon film or a polycrystalline semiconductor film. A crystalline silicon film formed by laser annealing has high mobility. Accordingly, it is actively used in, for example, monolithic type liquid crystal electro-optical devices where thin film transistors (TFTs) are formed using this crystalline silicon film and used as TFTs for driving pixels and driving circuits formed on one glass substrate.
Further, a method of performing laser annealing by processing a high output pulse laser beam, such as an excimer laser, by means of optical system into a square spot of several centimeters, or into a linear shape of 10 cm or more, on the surface to be irradiated, and scanning the laser beam (or by moving the laser beam irradiation position relatively to the surface to be irradiated), has been preferably used because it has a high degree of mass productivity and is industrially excellent.
In particular, if a linear shape laser beam is used, a high degree of mass productivity can be obtained because laser irradiation can be performed on the entire surface to be irradiated by merely scanning at a right angle to the longitudinal direction of the linear shape laser, unlike the case of using a spot shape laser beam in which it is necessary to scan forward, back, left, and right. Scanning is made at a right angle to the longitudinal direction because it is the most efficient scanning direction. On account of this high mass productivity, the use in laser annealing of a linear shape laser beam in which a pulse oscillation excimer laser beam is processed by a suitable optical system, is now becoming a mainstream.
A polycrystalline silicon film can be obtained for the case of processing the pulse emission excimer laser beam into a linear shape and irradiating the linear shape laser beam while scanning, for example, on a non-single crystal silicon film.
The consequences of carefully studying or examining the obtained polycrystalline silicon film lead to a known fact that the polycrystalline silicon film is formed of a single crystal made of an infinite number of crystal grains with a grain size of several hundred nanometers. A large number of lattice defects exist in the grain boundary of these crystal grains causing substantial damages to the characteristics of a semiconductor device.
This problem can be resolved by making the grain size larger so that the lattice defect density may be reduced.
Laser irradiation has been performed on the non-single crystal silicon film under various possible conditions to resolve the above problem. However, the average grain size of the polycrystalline silicon film obtained could not be larger than 500 nm.
For example, irradiating a laser under a nitrogen atmosphere, or under a vacuum state, or under a heated state. A method of combining these methods in which the conditions are optimized was also tried. In any case, the grain size could not be made markedly bigger by any of the methods.